Lithium batteries remain prevalent around the world as an electricity source within a myriad of products. From rechargeable power tools, to electronic cars, to the ubiquitous cellular telephone (and like tablets, hand-held computers, etc.), lithium batteries (of different ion types) are utilized as the primary power source due to reliability, above-noted rechargeability, and longevity of usage. With such widely utilized power sources, however, comes certain problems, some of which have proven increasingly serious. Notably, safety issues have come to light wherein certain imperfections within such lithium batteries, whether due to initial manufacturing issues or time-related degradation problems, cause susceptibility to firing potentials during short circuit events. Basically, internal defects with conductive materials have been found to create undesirable high heat and, ultimately, fire, within such battery structures. As a result, certain products utilizing lithium batteries, from hand-held computerized devices (the Samsung Galaxy Note 7, as one infamous situation) to entire airplanes (the Boeing 787) have been banned from sales and/or usage until solutions to compromised lithium batteries used therein and therewith have been provided (and even to the extent that the Samsung Galaxy Note 7 has been banned from any airplanes in certain regions). Even the Tesla line of electric cars have exhibited notable problems with lithium battery components, leading to headline-grabbing stories of such expensive vehicles exploding as fireballs due to battery issues. Widespread recalls or outright bans thus remain today in relation to such lithium battery issues, leading to a significant need to overcome such problems.
These problems primarily exist due to manufacturing issues, whether in terms of individual battery components as made or as such components are constructed as individual batteries themselves. Looked at more closely, lithium batteries are currently made from six primary components, a cathode material, a cathode current collector (such as aluminum foil) on which the cathode material is coated, an anode material, an anode current collector (such as copper foil) on which the anode material is coated, a separator situated between each anode and cathode layer and typically made from a plastic material, and an electrolyte as a conductive organic solvent that saturates the other materials thereby providing a mechanism for the ions to conduct between the anode and cathode. These materials are typically wound together into a can, as shown in Prior Art FIG. 1, or stacked. There are many other configurations that are and may be utilized for such battery production purposes, including pouch cells, prismatic cells, coin cells, cylindrical cells, wound prismatic cells, wound pouch cells, and the list goes on. These battery cells, when made correctly and handled gently, can provide energy for various applications for thousands of charge-discharge cycles without any appreciable safety incident. However, as alluded to above, certain events and, in particular, certain defects can cause internal shorting between the internal conductive materials which can lead to heat generation and internal thermal runaway, known to be the ultimate cause of fire hazards within such lithium batteries. Such events may further be caused by, as noted above, internal defects including the presence of metallic particles within the battery, burrs on the current collector materials, thin spots or holes in the separator (whether included or caused during subsequent processing), misalignments of battery layers (leaving “openings” for unwanted conductivity to occur), external debris penetrating the battery (such as road debris impacting a moving vehicle), crushing and/or destabilizing of the cell itself (due to accidents, for instance), charging the cell in a confined space, and the like. Generally speaking, these types of defects are known to cause generation of a small electronic conductive pathway between the anode and cathode. When such an event occurs, if the cell is then charged, such a conductive pathway may then cause a discharge of the cell therethrough which ultimately generates excessive heat, thereby compromising the battery structure and jeopardizing the underlying device being powered thereby. Combined with the presence of flammable organic solvent materials as battery electrolytes (which are generally of necessity for battery operability), such excessive heat has been shown to cause ignition thereto, ultimately creating a very dangerous situation. Such problems are difficult to control once started, at the very least, and have led to significant injuries to consumers. Such a potential disastrous situation is certainly to be avoided through the provision of a battery that delivers electrical energy while not compromising the flammable organic electrolyte in such a manner.
The generation of excessive heat internally may further create shrinkage of the plastic separator, causing it to move away from, detach, or otherwise increase the area of a short within the battery. In such a situation, the greater exposed short area within the battery may lead to continued current and increased heating therein, leading to the high temperature event which causes significant damage to the cell, including bursting, venting, and even flames and fire. Such damage is particularly problematic as the potential for firing and worse comes quickly and may cause the battery and potentially the underlying device to suffer an explosion as a result, putting a user in significant danger as well.
Lithium batteries (of many varied types) are particularly susceptible to problems in relation to short circuiting. Typical batteries have a propensity to exhibit increased discharge rates with high temperature exposures, leading to uncontrolled (runaway) flaring and firing on occasion, as noted above. Because of these possibilities, certain regulations have been put into effect to govern the actual utilization, storage, even transport of such battery articles. The ability to effectuate a proper protocol to prevent such runaway events related to short circuiting is of enormous importance, certainly. The problem has remained, however, as to how to actually corral such issues, particularly when component production is provided from myriad suppliers and from many different locations around the world.
Some have honed in on trying to provide proper and/or improved separators as a means to help alleviate potential for such lithium battery fires. Low melting point and/or shrinkage rate plastic membranes appear to create higher potentials for such battery firing occurrences. The general thought has then been to include certain coatings on such separator materials without reducing the electrolyte separation capabilities thereof during actual utilization. Thus, ceramic particles, for instance, have been utilized as polypropylene and/or polyethylene film coatings as a means to increase the dimensional stability of such films (increase melting point, for example). Binder polymers have been included, as well, as a constituent to improve cohesion between ceramic particles and adhesion to the plastic membrane (film). In actuality, though, the thermal increase imparted to the overall film structure with ceramic particle coatings has been found to be relatively low, thus rendering the dominant factor for such a separator issue to be the actual separator material(s) itself.
As a result, there have been designed and implemented, at least to a certain degree, separator materials that are far more thermally stable than the polyethylene and polypropylene porous films that make up the base layer of such typical ceramic-coated separators. These low shrinkage, dimensionally stable separators exhibit shrinkage less than 5% when exposed to temperatures of at least 200° C. (up to temperatures of 250, 300, and even higher), far better than the high shrinkage rates exhibited by bare polymer films (roughly 40% shrinkage at 150° C.), and of ceramic-coated films (more than 20% at 180° C.) (such shrinkage measurement comparisons are provided in Prior Art FIG. 2). Such low shrinkage rate materials may change the mechanism of thermal degradation inside a target cell when a short occurs. Generally speaking, upon the occurrence of a short within such a battery cell, heat will always be generated. If the separator does not shrink in relation to such a short circuit event, heat will continue to be generated and “build up” until another material within the battery degrades. This phenomenon has been simulated with an industry standard nail penetration test. For instance, even with a separator including para-aramid fiber and exhibiting a shrinkage stability up to 550° C., the subject test battery showed a propensity to short circuit with unique internal results. Such a cell was investigated more closely subsequent to such treatment wherein the cell was opened, the excess electrolyte was evaporated, the cell filled with epoxy and then sectioned perpendicular to the nail, which was left in the cell. Scanning electron microscope images were then undertaken using backscattered electron imaging (BEI), which enabled mapping of the different battery elements to show the effect of such a nail penetration activity. These are shown in Prior Art FIGS. 3a and 3b. 
In Prior Art FIG. 3a, it is noted that the copper layers consistently come closer to the nail than the aluminum layers. It is also noted that the high stability separator is still intact between the electrodes. Prior Art FIG. 3b shows a higher magnification of the end of one aluminum layer, showing that it ends in a layer of cracked grey matter. This was investigated with BEI, which showed the resultant matter to actually be aluminum oxide, an insulating ceramic. Such evidence led to the proposed conclusion that when the separator itself is thermally stable, the aluminum current collector will oxidize, effectively breaking the circuit (and stopping, as a result, any short circuit once the insulating aluminum oxide is formed). Once the circuit is broken, the current stops flowing and the heat is no longer generated, reversing the process that, with less stable separators, leads to thermal runaway.
This possible solution, however, is limited to simply replacing the separator alone with higher shrinkage rate characteristics. Although such a simple resolution would appear to be of great value, there still remains other manufacturing procedures and specified components (such as ceramic-coated separator types) that are widely utilized and may be difficult to supplant from accepted battery products. Thus, despite the obvious benefits of the utilization and inclusion of thermally stable separators, undesirable battery fires may still occur, particularly when ceramic coated separator products are considered safe for such purposes. Thus, it has been determined that there is at least another, solely internal battery cell structural mechanism that may remedy or at least reduce the chance for heat generation due to an internal short in addition to the utilization of such highly thermal stable separator materials. In such a situation, the occurrence of a short within such a battery cell would not result in deleterious high temperature damage due to the cessation of a completed internal circuit through a de facto internal fuse creation. Until now, however, nothing has been presented within the lithium battery art that easily resolves these problems. The present disclosure provides such a highly desirable cure making lithium battery cells extremely safe and reliable within multiple markets.